The seven are Bang Sue, Chatuchak, Phayathai, Din Daeng, Huay Kwang, Dusit and Phra Nakhon districts.
They could be hit by light to middle-level rains from 3pm to 9pm, the BMA warned.
On Wednesday morning, the Meteorological Department forecast that heavy rains would fall in the next 24 hours in the North, the Central region including Bangkok and suburban areas, the East and the South.
The department advised people living on the banks of canals and rivers to brace for possible water overflows.